Page 356 - Mechanics Analysis Composite Materials

Chapter 7. Environmental. special loading, and manufacturing eflects 34I

1

0.8

0.6

0.4

0.2

0
0 0.2 0.4 0.6 0.8 1
Fig. 7.32. Dependence of the tangent modulus normalized to its initial value on the number of cycles
related to the ultimate number correspondingto fatigue failure under stress umsx= 120 MPa and R = -1
for f45"angle-ply carbon-epoxy laminate.
laminates that sometimescan be hardly identified by visual examination. This type of
the defect causesa dramatic reduction in the laminate compressivestrength and results
in unexpected failure of the thin-walled composite structure due to microbucklingof
fibers or local buckling of plies. As follows from Fig. 7.33 showing experimental
results of Verpoest et al. (1989) for unidirectional and fabric composite plates,
impact can reduce material strength in compression by the factor of 5 and more.
To study the mechanism of material interlaminar delamination, consider a
problem of wave propagation through the thickness of the laminate shown in
Fig. 7.34. The motion equation has the following well-known form

(7.58)

Here, u, is the displacement in the z-direction, E, is material modulus in the same
direction depending, in the general case on z, and p is the material density. For the
laminate in Fig. 7.34, the solution of Eq. (7.58) should satisfy the following
boundary and initial conditions

r&(Z = 0,t) = -p(t), az(z= h, t) = 0 , (7.59)
au,
uz(z,t = 0) = 0, -(z > 0,t = 0) = 0 (7.60)
at
in which

(7.61)

is the interlaminar normal stress.